(1) Field of the Invention
This invention relates to an LED (Light-Emitting Diode) driving circuit and a feedback control circuit thereof, and more particularly relates to an LED driving circuit with precise dimming control and a feedback control circuit thereof.
(2) Description of the Prior Art
Because of the properties of long lifetime, high luminance efficiency, and fast and steady illumination, etc., an LED has been broadly accepted as a main trend of light sources for the next generation in recent years. The LEDs can be used in various applications, including indoor lighting, outdoor lighting, and commercial advertisement lighting, etc., and thus the existing light sources are gradually replaced by the LEDs. It is an important issue regarding how to make the LEDs generate illumination with steady brightness and uniform color and to provide proper protection to the LEDs so as to exhibit the lighting advantages of the LEDs.
FIG. 1 is a circuit diagram of a typical LED driving circuit. As shown in FIG. 1, the LED driving circuit includes a feedback control circuit 100, a converting circuit 110, and an LED module 120. The converting circuit 110 is coupled to an input power source VIN for converting the input power source VIN into an output voltage VOUT to drive the LED module 120 for illumination. The conversion operation performed by the converting circuit 110 may be a step-up conversion or a step-down conversion. Take a DC-to-DC boost converting circuit as an example. The converting circuit 110 includes an inductor L1, a transistor SW1, a rectifying diode K1, and an output capacitor C1. The inductor L1 has one end coupled to the input power source VIN and the other end coupled to the transistor SW1, and an inductor current IL1 flows through the inductor L1. The transistor SW1 has one end coupled to the inductor L1 and another end coupled to the ground through a resistor R1. The output capacitor C1 has one end coupled to a junction between the inductor L1 and the transistor SW1 through the rectifying diode K1 and the other end grounded. The LED module 120 has a plurality of LED strings connected in parallel. To make sure a substantially identical current flowing through each of the LED units in the LED module 120, a current balancing unit 130 with a plurality of current balancing ends D1˜Dn coupled to the corresponding LED strings in the LED module 120 is used for balancing the current of each of the LED strings, so as to have the current stabilized at a predetermined current value. The driving voltages of the current balancing ends D1˜Dn should be maintained at or above a lowest operable voltage level to make sure that the current balancing unit 130 works normally. For detecting the driving voltage, a voltage detecting circuit 140 is used and is coupled to the current balancing ends D1˜Dn for detecting the level of the current balancing ends D1˜Dn, which would be varied in response to the variations of voltage difference on the LED strings through while a current with the predetermined current value flows. To have the current balancing ends D1˜Dn at or above a lowest operable voltage level, the voltage detecting circuit 140 generates a feedback signal Fb1 according to the level of the current balancing end which has the lowest level among all the current balancing ends D1˜Dn. The feedback control circuit 100 controls the converting circuit 110 to generate the output voltage VOUT according to the feedback signal Fb1 to maintain all the current at or above the predetermined current value. The current balancing unit 130 also receives a dimming signal DIM and starts or stops the current flowing through the LED module 120 according to the dimming signal DIM for the burst dimming control. The voltage detecting circuit 140 may have a plurality of diodes, and each diode has a negative end coupled to the corresponding current balancing end D1˜Dn and a positive end coupled to a common driving power source VCC through the same resistor.
The feedback control circuit 100 includes a feedback unit 150 and a pulse width control unit 160. The feedback unit 150 includes an amplifying unit 152 and a compensation unit 154. The amplifying unit 152 receives the feedback signal Fb1 and a reference signal Vr1 so as to generate an output signal. The output signal is then compensated by the compensation unit 154, so as to generate a pulse width control signal Vea1. The pulse width control unit 160 includes a pulse width modulation unit 162 and a driving unit 164. The pulse width modulation unit 162 receives the pulse width control signal Vea1 and a ramp signal so as to generate a pulse width modulation signal S1. The driving unit 164 receives the pulse width modulation signal S1 and the dimming signal DIM, and accordingly generates a control signal Sc1.
Due to the capacitor of the compensation unit 154, the level of the pulse width control signal Vea1 is varied slowly. Referring to FIG. 2, the peak value of the inductor current IL1 on the inductor L1 cannot be rapidly increased in response to the dimming signal DIM, which may result in the insufficiency of power transferred from the converting circuit 110 to the LED module 120. Right after the dimming signal DIM is changed to “ON” state, the output voltage VOUT may drop to an extremely low level first and then rise to converge on a stable level. The low level output voltage VOUT may result in a driving current which is smaller than the predetermined current value within a period right after the state of the dimming signal DIM is changed from the “OFF” state to the “ON” state, such that precision of dimming control for the LED module is badly influenced.